1. Field of the Invention
The present invention relates to power supply apparatuses and image forming apparatuses using the power supply apparatuses.
2. Description of the Related Art
In conventional electrophotographic image forming apparatuses, a wire-wound electromagnetic transformer has been used as a power supply apparatus to generate a high voltage required in image forming processes, such as transfer and the like. In the power supply apparatus using the electromagnetic transformer, for example, a transformer winding needs to be insulated using a mold or the like, and a relatively large transformer is required, whereby the reduction of size and weight of the power supply apparatus is prevented. Therefore, a piezoelectric transformer that is used as a power supply apparatus to generate a high voltage has been studied as an alternative to the electromagnetic transformer (see Japanese Patent Laid-Open No. 11-206113). Piezoelectric transformers, which are made of ceramics, have a thin thickness and a light weight, and are capable of generating a high voltage with higher efficiency than that of the electromagnetic transformer. Also, piezoelectric transformers do not require a molding process for insulation that is required for the electromagnetic transformer, and therefore, are useful in achieving a reduction in size and weight of a power supply apparatus that can output a high voltage.
Piezoelectric transformers have specific resonance characteristics that are determined based on the structural characteristics, such as dimensions and the like, and output from the secondary side a voltage according to the frequency of a drive signal input to the primary side. For example, a voltage-controlled oscillator (VCO) is coupled to the piezoelectric transformer, and the frequency of a drive signal output from the VCO is varied, thereby varying the output voltage of the piezoelectric transformer. By thus controlling the piezoelectric transformer, the output voltage of the piezoelectric transformer can be set to a target voltage.
In general, when piezoelectric transformers are used as a power supply apparatus, the frequency range of the drive signal from the VCO is set to a range including a resonance frequency in order to obtain a predetermined range of output voltages from the piezoelectric transformer where the output voltage takes the highest value when the piezoelectric transformer resonates. Also, by changing the frequency of the drive signal from the VCO, the output voltage of the piezoelectric transformer is set to a target voltage. In order to set the output voltage of the piezoelectric transformer to a target voltage, the VCO determines the amount of change in the frequency of the drive signal output to the piezoelectric transformer based on, for example, a difference value between the output voltage fed back from the piezoelectric transformer and the target voltage. Specifically, the VCO decreases the amount of change in the frequency of the drive signal when the difference value is small, and increases the amount of change in the frequency of the drive signal when the difference value is large. By thus changing the drive frequency of the VCO, the output voltage of the piezoelectric transformer can be caused to approach the target voltage in a stepwise manner.
The above conventional techniques, however, have the following problems. For example, if frequency characteristics corresponding to the relationship between frequencies and output voltages of the piezoelectric transformer indicate that a spurious frequency component is present in frequency components other than the resonance frequency, a rise time that is required for controlling the output voltage of the piezoelectric transformer to a target voltage by changing the frequency of the drive signal, disadvantageously increases. Specifically, when the VCO changes the frequency of the drive signal from an initial frequency to a target frequency corresponding to the target voltage, then if a spurious frequency is present between the initial frequency and the target frequency, the output voltage of the piezoelectric transformer temporarily increases at the spurious frequency. In this case, the difference value between the output voltage and the target voltage of the piezoelectric transformer decreases, and therefore, the amount of change in the drive frequency temporarily decreases. As a result, the rise time that it takes to change the drive frequency to the target frequency disadvantageously increases.